1. Field of the Invention
The present invention relates generally to optical systems, and more particularly to beam expanding assemblies, line narrowing modules, lasers, and other optical systems having desirable beam wavefront characteristics.
2. Technical Background
For many applications, optical systems that provide beams of radiation having specified wavefront properties are highly desired. For example, in the UV photolithography processes used to make semiconductor devices such as computer chips, a substantially planar wavefront is highly desirable in order reduce interferometric effects and to achieve the best possible photolithographic performance. Further, the performace of the UV lasers used in these photolithographic processes is dependent on the wavefront characteristics of the beam propagating inside the laser, which are in turn dependent on the optical elements of the laser. The optical elements of the laser are generally not ideal, due, for example, to manufacturing and material limitations. The wavefront characteristics of the beam have a strong effect on the properties of the laser output. For example, the linewidth and center wavelength of the laser can be negatively affected when the beam propagating inside the laser has a change in wavefront profile. Further, the linewidth and center wavelength of the laser can change as the laser warms up from the time it is turned on to the time it reaches thermal equilibrium. This thermal drift can be especially problematic in high power UV lasers, which use CaF2 optical elements. While CaF2 has a highly desirable transparency for use with radiation below 300 nm, it has a relatively high coefficient of thermal expansion (about 18.9 ppm/K at room temperature).
In order to improve the optical performance of UV lasers, one current approach is to insert adaptive optics or bendable gratings into the line narrowing module conventionally used to narrow the natural linewidth of the laser. While this approach can be effective, such actively adjustable elements can be very expensive, require complicated driving mechanisms, and can be difficult to combine with the rest of the stringently engineered laser architecture. Further, such actively adjustable elements can tend to increase the risk of failure and instability, and may not be able to sufficiently correct static and dynamic wavefront errors in the laser.